An alarm hazard indicator, e.g., a fire alarm or a security alarm, communicates with a control center either via a two-core line or wirelessly. The indicator typically includes at least one sensor sensitive to a physical variable (e.g., smoke) and a signal processing circuit that activates a high-power LED. The signal processing circuit may activate the LED not only during an alarm, but also during a test operation triggered by the control center. The indicator typically receives its supply voltage via the two-core line (from the control center) or from an installed battery.
Since the power received by the indicator is limited, the LED is often operated via a blink cycle. In a blink cycle, the LED has a frequency of 1 Hz and a short “ON” time (e.g., in the range of approximately 30 ms), The LED may be turned on and off in a rapid pulse sequence in this “ON” time, (e.g., having a pulse duration of 20 μs per pulse period of 200 μs). Regulations, however, are increasingly mandating the LED (e.g., a red LED intrinsic to the indicator) must be illuminated continuously and at a brightness level significantly greater than the operating brightness of the conventional LEDs (e.g., LEDs typically used only for indicating the operating state (e.g., idle, test, and alarm)).
These conventional red LEDs, however, are capable of developing sufficiently high luminosity at higher current level (e.g., a current of about 3 mA). In contrast, typical control centers and indicator lines connected thereto, having up to 256 indicators linked to the control center, are designed for indicators that consume about 100 μA in the idle and/or readiness state at a line voltage of 18 to 19 V. If an indicator having such an LED consumed approximately 3 mA in the alarm state, the line voltage would drop so strongly that only a few indicators in close physical proximity to the control center would function. Consequently, to operate the LEDs “continuously”, the above-described blink cycle must be used, since this rapid pulse sequence is perceived like a continuous light by the human eye.
For example, assuming a line voltage of 19 V, for example, an on-state voltage of the LED of 1.6 V, and an ideal switching regulator (having an efficiency of 100%), the indicator would theoretically only still consume approximately 250 μA. The actual current consumption is much higher, however, not only because of the real efficiency of the switching regulator, but also because of the current consumption of the control circuit, which is required for limiting the peak current through the LED to a permissible highest value. This control circuit includes an operational amplifier in a conventional indicator, which has a current consumption of approximately 1 mA during every pulse. This current consumption is tolerable in conventional indicators, since the LED is operated using a current of a few hundred mA, and since the operational amplifier only operates within the short “ON” time of approximately 30 ms (being unpowered during the subsequent pause of approximately 970 ms). For an indicator whose LED is instead to be illuminated continuously for the duration of the alarm state, however, this additional current consumption of the control circuit of the known indicator is not acceptable because its current consumption in the alarm state would thus increase to approximately 1.5 mA, with the result that the number of indicators usable per indicator line would drop very significantly.